1. Field of the Invention
The present invention relates to the field of electronic reproduction technology, and in particular to a method and to an apparatus for the correction of positional errors of a light beam deflected across a surface point and line-wise by deflection system having at least one mirrored surface, the light beam moving at right angles to the deflection direction relative to the deflection system, and in which method positional errors are determined and are corrected by corresponding correction values.
2. Description of the Prior Art
Light beam deflection systems are employed in reproduction technology, for example, for the point and line-wise master scanning or for recording information.
In the case of scanning, a light beam is deflected point and line-wise across a master by a polyhedral rotating mirror. The scanning light reflected by the master is converted into an image signal by an opto-electronic transducer.
In the case of information recording, the light beam is intensity-modulated as a function of an image signal and is deflected point and line-wise across a recording medium (film) by a polyhedral rotating mirror. The master or the recording medium move either continuously or by steps at right angles to the deflection direction of the light beam.
In order to provide good reproduction quality, it is required, among other things, that the line spacings be identical, and that the line beginnings and line ends respectively lie precisely over one another. Due to optical and geometrical errors of the polyhedral rotating mirror and the positional errors of the light beam on the surface which are caused thereby, the reproduction quality, however, is considerably decreased.
The causes of these errors, for example, are the tolerances in the manufacture of the polyhedral rotating mirror and the deficiencies in the structure of the arrangement. Mirror surfaces which are not aligned parallel to the axis of rotation of a polyhedral rotating mirror and an unstable position of the axis of rotation cause positional errors on the surface at right angles to the deflection direction of the light beam which result in non-uniform line spacings.
Differing angles between adjacent mirror surfaces cause non-coincidence between the line beginnings and the line ends.
It has been shown, in practice, that mirror surfaces which are not flat and are not planar have a disruptive effect if a very high accuracy is to be achieved. Due to uneven mirror surfaces, the positional errors at right angles to the deflection direction become dependent on the deflection direction of the light beam or, respectively, on the line direction. Therefore, the light beam has a non-uniform angular velocity in the line direction as it does given a non-constant rotational speed of the polyhedral rotating mirror, as a result of which distortions occur within the lines. In addition, the described errors can change due to thermal effects or due to aging.
A correction arrangement for the positional errors of a polyhedral rotating mirror is already known from the U.S. Pat. No. 4,002,830 in which a correction deflector is disposed in the beam path between the light source and the polyhedral rotating mirror. In this arrangement, only one correction value is assigned to each mirror surface. Measures for measuring the positional errors and for generating correction values are not set forth.
A further correction arrangement is disclosed in the German published application No. 29 27 101 in which the positional errors at right angles to the deflection direction are corrected by deflector and the positional errors in the deflection direction are corrected by regulating the speed of a polyhedral rotating mirror. In this arrangement, also, only one correction value is generated for each mirror surface.
The known correction arrangements have the disadvantage that errors caused by non-planar surfaces are not corrected.